1. Field of the Invention
The present invention relates to a magnetic shield structure for a color cathode ray tube, and in particular to an improved inner shield structure which can shield all sides of a tension mask assembly.
2. Description of the Related Art
In general, a color cathode ray tube includes a flat panel 1 composing a screen; a funnel 2 disposed at the rear portion of the panel 1, an electron gun being inserted into its neck unit; and a deflecting yoke 5 disposed at the outer circumferential surface of the neck unit, for deflecting an electron beam 6 scanned from the electron gun.
Here, a fluorescent surface 4 for coating red, green and blue fluorescent materials on a black matrix of the panel 1 is positioned in the funnel 2 and the panel 1, and a tension mask 3 is positioned separately from the fluorescent surface 4, so that the electron beam 6 scanned from the electron gun can be color discriminated and landed on the fluorescent surface 4.
The tension mask 3 is supported by a main frame 14 and a sub frame 13, thus composing a tension mask assembly. The tension mask assembly is fixed by stud pins disposed at the respective inner side portions of the panel 1. An inner shield 9 is provided at the rear portion of the frames so that the electron beam 6 can pass through the funnel 2, not distorted by an earth magnetic field.
As illustrated in FIGS. 2A and 2B, the conventional inner shield 9 includes: a beam shield 17 disposed at the rear portion of the main frame 14, for shielding the rear portion of the main frame 14; and a shield 18 for shielding the inside of the funnel, so that the path of the electron beam passing the funnel cannot be distorted.
Here, a section of the shield 18 is formed in a trapezoidal shape having no top and bottom surfaces. The shield 18 may be incorporated with the beam shield 17, or connected with the beam shield 17 by fixing pins.
On the other hand, in the structure where the sub frame 13 and the main frame 14 are welded, the tension mask 3 is welded on the main frame 14 by applying force to the sub frame 13, and the force applied to the sub frame 13 is removed, thus fabricating the tension mask assembly.
Accordingly, when the inner shield 9 is disposed at the rear portion of the tension mask assembly, the main frame 14 and the tension mask 3 are not influenced by the earth magnetic field due to direct welding. However, since the sub frame 13 and the tension mask 3 are not connected, a side space 19 between the main frame 14 and the tension mask 3 is exposed to the earth magnetic field.
Referring to FIGS. 3A and 3B, another conventional inner shield 9 includes: a beam shield 17 having a protrusion for shielding a frame at its lower portion; and a shield 18 having its section formed in a trapezoidal shape to shield the inside of the funnel.
The inner shield 9 is also disposed at the rear portion of the tension mask assembly, so that the beam shield 17 can shield the inner sides and rear portion of the main frame and the shield 18 can shield the inner sides of the funnel. However, a side space 19 between the main frame and the tension mask is exposed to the earth magnetic field.
Accordingly, when the color cathode ray tube having the inner shield 9 is operated to scan the electron beam into the panel, as shown in FIG. 4A, the electron beam 6 is distorted due to the earth magnetic field in passing through the funnel and a slot of the tension mask 3, and thus landed on a different fluorescent material.
That is, when the electron beam 6 is scanned to the whole regions of the panel, the electron beam 6 is deflected due to the deflecting yoke in a horizontal or vertical direction, and thus landed on an original position. However, in the case that a direction of the electron beam is changed by the deflecting yoke or the external earth magnetic field is varied, the path of the electron beam is distorted, and thus the electron beam is landed on an after-movement position.
As a result, as shown in FIG. 4B, the electron beam 6 is not landed on the original fluorescent material 16, but mistakenly landed on the black matrix 15 coated between the fluorescent materials 16. In addition, a different color fluorescent material 16 on the after-movement position may be emitted due to distortion of the path of the electron beam 16, thereby reducing chrominance purity of the screen.
Moreover, the main frame, sub frame and beam shield for shielding the earth magnetic field consist of a material having low permeability which is inversely proportional to magnetic flux strength and proportional to magnetic flux density, and thus the path of the electron beam is distorted.
In addition, the electron beam is reflected from the main or sub frame, to cause halation on the screen. Therefore, the screen is dimmed.
The rotation redundancy and magnetic field redundancy for deflecting the electron beam are restricted due to the halation. It is thus difficult to improve color chrominance of images.
Accordingly, an object of the present invention is to provide a color cathode ray tube which can improve color chrominance, rotation redundancy and magnetic field redundancy, by providing a magnetic shield structure for landing an electron beam on a designated fluorescent material without distorting a path of the electron beam.
In order to achieve the above-described object of the present invention, in a color cathode ray tube including a tension mask assembly consisting of a tension mask for color-discriminating an electron beam, and a main frame and a sub frame for supporting the tension mask, and a magnetic shield structure disposed at a funnel, for preventing deflection and distortion of the electron beam, the magnetic shield structure includes a main unit for shielding the inner sides of the funnel; and a front unit where the tension mask assembly is inserted.
Preferably, the front unit has a hole so that the tension mask assembly can be inserted into the hole. Both end portions of the front unit have side walls.
More preferably, the main unit has an electron beam passing hole, and is formed in a barrel shape corresponding to the inside shape of the funnel.
In addition, the front unit has at its both end portions an internally-protruded inside wall, and an outside wall for forming an outer wall separated from the inside wall by a top surface having a predetermined thickness.